The present invention relates to a cathode element of an electron gun of a cathode ray tube, and more particularly of a color television picture tube. The electron beam raster of a color television picture tube is formed by indirectly heating an emissively-coated cathode member to produce electron emission which is accelerated toward a high voltage anode. A finite time is required to heat the electron emissively-coated member to a stable operating temperature, at which time the electron emission forms a stable raster beam. This finite heating time is responsible for a delay from the turn on of the set to the production of a display image upon the picture tube. This warm-up time has characteristically been of the order of about 20 to 30 seconds.
The on-set time of the display image has been minimized in the past by continuously heating the cathode heater to keep the electron gun cathode at or near operating temperature even while the set is turned off. This feature has the disadvantage of being inefficient energy wise, and presents somewhat of a safety hazard.
It is desirable to be able to turn on the set and have the cathode warm up to operating temperature and produce a visible image in as short a period of time as possible. One approach to providing such a fast warm-up cathode characteristic has been to insulate the emissive portion of the cathode from the bulk of the cathode structure and electron gun structure to minimize the heat dissipation from the heated portion of the cathode. This involves fabrication of a relatively complicated cathode which is expensive to manufacture and requires an insulator stand-off means between the emissive cathode portion and the support structure. Still other fast warm-up cathodes have been proposed in U.S. Pat. No. 3,333,138, which eliminates the need for insulating stand-off means. A plurality of thin conductive support straps connect the cathode cap to the cathode support sleeve. This structure is difficult to manufacture because of the need to properly align and connect these thin support straps, which must accurately maintain the cathode cap in predetermined position relative to the control electrodes of the electron gun.